Here's the thing with comparing Chuck Liddell to GSP.
There was always someone out there who could beat Chuck - Rampage.
There's someone out there who can beat Anderson Silva (IMO) - Sonnen or even GSP
There's absolutely no one out there that I can see beating GSP.
Chuck's decline was due to better fighters existing, him getting old and him losing his chin from a long and tough career. GSP hasn't taken that much damage considering how long he's been fighting, he's not that old, and it doesn't matter if his chin somehow goes because he doesn't even get hit.
I'm most curious about how GSP will fall. Every fighter does, but maybe he'll be the first to retire without losing at WW. After Shields there's no one left. It would take years for the next wave of evolved fighters to materialize, GSP will be retired or at MW by then.